1. Field of the Invention
The present invention generally relates to completion operations in a wellbore. More particularly, the invention relates to a packer for sealing an annular area between two tubular members within a wellbore. More particularly still, the invention relates to a packer having a bi-directionally boosted and held packing element.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the formation. A cementing operation is then conducted in order to fill the annular area with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. The well is then drilled to a second designated depth, and a second string of casing, or liner, is run into the well. The second string is set at a depth such that the upper portion of the second string of casing overlaps with the lower portion of the upper string of casing. The second “liner” string is then fixed or “hung” off of the upper surface casing. Afterwards, the liner is also cemented. This process is typically repeated with additional liner strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
The process of hanging a liner off of a string of surface casing or other upper casing string involves the use of a liner hanger. In practice, the liner hanger is run into the wellbore above the liner string itself. The liner hanger is actuated once the liner is set at the appropriate depth within the wellbore. The liner hanger is typically set through actuation of slips which ride outwardly on cones in order to frictionally engage the surrounding string of casing. The liner hanger operates to suspend the liner from the casing string. However, it does not provide a fluid seal between the liner and the casing. Accordingly, it is desirable in many wellbore completions to also provide a packer.
During the wellbore completion process, the packer is run into the wellbore above the liner hanger. A threaded connection typically connects the bottom of the packer to the top of the liner hanger. Known packers employ a mechanical or hydraulic force in order to expand a packing element outwardly from the body of the packer into the annular region defined between the packer and the surrounding casing string. In addition, a cone is driven behind a tapered slip to force the slip into the surrounding casing wall and to prevent packer movement. Numerous arrangements have been derived in order to accomplish these results.
A disadvantage with known packer systems is the potential for becoming unseated. In this regard, wellbore pressures existing within the annular region between the liner and the casing string act against the setting mechanisms, creating the potential for at least partial unseating of the packing element. Generally, the slip is used to prevent packer movement also traps into the packer element the force used to expand the packer element. The trapped force provides the packer element with an internal pressure. During well operations, a differential pressure applied across the packing element may fluctuate due to changes in formation pressure or operation pressures in the wellbore. When the differential pressure approaches or exceeds the initial internal pressure of the packer element, the packing element is compressed further by the differential pressure, thereby causing it to extrude into smaller voids and gaps. Thereafter, when the pressure is decreased, the packing element begins to relax. However, the internal pressure of the packer element is now below the initial level because of the volume transfer during extrusion. The reduction in internal pressure decreases the packer element's ability to maintain a seal with the wellbore when a subsequent differential pressure is applied.
Therefore, there is a need for a packer system in which the packing element does not disengage from the surrounding casing under exposure to formation pressure. In addition, a packer system is needed in which the presence of formation pressure only serves to further compress the packing element into the annular region, thereby assuring that formation pressure will not unseat the seating element. Further still, a packer system is needed to maintain the internal pressure at a higher level than the differential pressures across the packer element. Further still, a packer system is needed to boost the internal pressure of the packer element above the differential pressure across the packer element. Further still, a packer system is needed that can boost the internal pressure of the packer element with equal effectiveness from differential pressure above or below the packer element.